Acoustic sounding instrument

ABSTRACT

Acoustic sounding instrument for use in determining the acoustic characteristics of the terrain of a borehole includes at least one acoustic emitter and at least one acoustic receiver disposed with the mean cross-sectional planes thereof disposed essentially parallel to the axis of the borehole, the emitter and receiver each being provided with a surface which is essentially a cylindrical surface of revolution through which the waves are emitted and received, respectively.

United States Patent [191' Lebreton Jan. 22, 1974 ACOUSTIC SOUNDINGINSTRUMENT [75] Inventor: Francisque Lebreton, Paris, France [73]Assignee: Institut Francais du Petrole des Carburants et Lubrifiants,Rueil-Malmaison, Hauts-de-Seine, France 22 Filed: Sept.21, 1971 211App]. No.: 182,458

[30] Foreign Application Priority Data Sept. 21, 1970 France 70.34200[52] U.S. Cl 181/.5 BE, 181/.5 P, 181/.5 ED [51] Int. Cl G01! 1/40 [58]Field of Search... 181/.5 BE, .5 R, .5 P, 5 ED; 3l0/8.6; 340/10 [56]References Cited UNITED STATES PATENTS 2,595,241 5/1952 Goble. 13 11.5BE

3,511,334 5/1970 Zemanek 181/.5 BE

OTHER PUBLICATIONS Brush Electronics Co., Pie zotronic Technical Data,"p. 4, 1953.

Primary Examiner-Benjamin A. B OrCheIt Assistant Examiner-J. V. DoramusAttorney, Agent, or FirmCraig et al.

[ 5 7 ABSTRACT Acoustic sounding instrument for use in determining theacoustic characteristics of the terrain of a borehole includes at leastone acoustic emitter and at least one acoustic receiver disposed withthe mean crosssectional planes thereof disposed essentially parallel tothe axis of the borehole, the emitter and receiver each being providedwith a surface which is essentially a cylindrical surface of revolutionthrough which the waves are emitted and received, respectively.

4 Claims, 11 Drawing Figures PATENTEDmzemn SHEEI 1 BF 2 FBG.1

FIGHZA FIGTA PATENTH] JAN 22 I974 SHEET 2 BF 2 ACOUSTIC SOUNDINGINSTRUMENT The present invention relates to an acoustic sounding orprobing instrument designed for receiving data pertaining to theacoustic characteristics of the terrain traversed by a borehole during adrilling operation.

The presently available equipment providing for the transmission andreception of acoustic waves make use of pairs of emitter-receiversimmersed in the liquid which generally fills the borehole, this liquidserving as a medium for transmitting the acoustic waves and beingindispensable to the receipt of a signal by the receiver after havingtraversed the terrain in question which has an amplitude sufficient toallow for the detection thereof.

The emitter and the receiver are generally placed in an enclosure madeof rubber and filled with oil so that the emitter and the receiver willin no case touch the wall directly. I

As a first approximation, the borehole may be considered as a cylinderof revolution. The emitter and the receiver having a low height (forexample, 2 to 3 centimeters) with respect to the diameter thereof (whichis in the order of from 8 to 10 centimeters) have an active surface withan essentially cylindrical configuration and a central axis which iscustomarily placed in coincidence with that of the borehole. This activesurface emits waves which, after arriving on the wall of the borehole,can be compared to the sperhical waves centered on the axis of theemitter only insofar as the emitter is centered in the borehole and hasa small dimension as compared to the cross section of the borehole.Inasmuch as these conditions are not met in actual practice because ofthe dimensions required for the emitter at the usual emissionfrequencies and energy, the active surface of the emitter emits in factnon directive waves issued from points situated on the lateral wall ofthe emitter cylinder.

Since the signals are emitted in the form of separate impulses, onemeasurement is carried out for each impulse, and for each measurementone beam of acoustic rays having a specific angle of incidence on thewall of the hole, as a function of the nature of the terrain, isutilized; while, the rays whose angle of incidence is different will notarrive at the receiver at all.

Under these conditions, points on the wall of the shaft are coincidentin phase over a height equal to the height of the emitter cylinder, thisheight being intersected by the beam of parallel rays. Inasmuch as thisheight is not negligible with respect to the wave length transmitted inthe terrain parallel to the wall, there are produced disturbinginterferences in the transmission, and these interferences areaccentuated at the receiver, irrespective of the nature of the terrainat the level of the receiver.

These defects, which become particularly apparent when one examines thegeneral aspect of the signal being received, will be aggravated when thehole is not in revolution, and/or when the distribution of thecharacteristics of the terrain does not display a symmetry of revolutionaround the axis of the hole, and/or when the axis of the sounding orprobing tool does not coincide with that of the hole, which increasesthe phase shift at the receiver.

These drawbacks (except for the sensitivity to a dissymmetry ofdistribution of the acoustic characteristics of the terrain around theaxis of the hole) are encountered again when acoustic transducers(emitters and receivers) are used whose emitting or receiving surface isconstituted principally of a plane face of the transducer, for example,the metallized end face of a ceramic cylinder.

These defects are eliminated according to the present invention with theaid of a sounding instrument comprising at least one acoustic emitterand at least one acoustic receiver which are spaced with respect to eachother, the emitter comprising an active element emitting in the groundacoustic signals principally following the radial directions of asurface having an essentially cylindrical body of revolution, and thereceiver receiving principally the acoustic signals following the radialdirections of the surface having an essentially cylindricalconfiguration, this sounding or probing instrument as proposed beingcharacterized in that the aforementioned cylindrical emitting surfaceand the aforementioned cylindrical receiving surface have the meancross-sectional planes thereof disposed essentially following the samelongitudinal plane of the sounding or probing instrument.

According to one embodiment of the sounding instrument proposed by thepresent invention, which is adapted to dip angle measurements of thelayers of ground, the sounding or probing instrument comprises threeemitters at a first level thereof and three receivers at a second levelthereof, and the aforementioned mean cross-sectional planes of theemitters and of the receivers are disposed essentially along threeplanes forming therebetween angles of Obtained thereby is an acousticdip-angle meter; in other words, a dip-angle meter whose operation doesnot require the use of the mud or sludge for conducting the electricity,which is in contrast to conventional dip-angle meters.

In a sounding instrument such as proposed by the present invention,there will be used advantageously acoustic transducers whose activeelement is a cylindri cal ring secured along the axis thereof betweentwo circular plates or supports with the interposition of annularfluid-tight seals.

The present invention will now be described hereinafter in a moredetailed fashion with reference to the accompanying drawings, wherein:

FIG. I is a schematic diagram of a type of sounding or probinginstrument used in the prior art;

FIG. 1A is a vertical cross-sectional view through the device shown inFIG. 1;

FIG. 2 is a schematic diagram of one example of a sounding instrument asproposed by the present invention;

FIG. 2A is a vertical cross-sectional view of the sounding instrumentshown in FIG. 2;

FIGS. 3 and 3A are, respectively, a cross-sectional view and anelevational view of one embodiment of the sounding instrument proposedby the present invention, adapted for the measurement of the dip angleof layers of terrain;

FIGS. 4 to 6 are diagrams of different types of acoustic transducers ofknown types, usable for forming the emitter and the receiver in asounding instrument according to the present invention; and

FIGS. 7 and 7A are an axial sectional view and a top view, respectively,of one example of an acoustic transducer in a sounding instrumentaccording to the present invention.

In FIGS. 1 and IA, which represent very schematically an arrangementadopted in the prior art for use as an acoustic sounding or probinginstrument, reference symbol T designates the emitter while referencesymbol R represents the receiver, and these acoustic transducers, whichare spaced with respect to each other, can be provided in any number.

The wall of the well is designated with reference numeral 6. Theaforementioned emitter and the aforementioned receiver have an activesurface which has an essentially cylindrical general configuration (oneof the shapes illustrated in FIGS. 4 to 7), with an axis parallel to theaxis of the well.

The acoustic signals are emitted by transmitter T in the form ofseparated impulses and for each impulse one beam of acoustic rays havinga well-defined angle of incidence i as a function of the ratio of thespeeds of propagation of the signals in the drilling mud and in theterrain at the level of the emitter T will be received by receiver Rafter penetration thereof into the ground where it is refracted by anangle r. The term acoustic ray is employed here-and in the entire texthereinafter for the purpose of designating in reality a pencil-likeacoustic ray having a very small opening angle, while the angles i and rcorrespond to the mean rays of the different incident and, respectively,reflected pencillike beams of rays. The rays 1 and 2, l and 2' are theextreme rays of this beam in the plane of FIG. I.

Since these rays reach the wall of the well in phase over the entireband of the wall, whose width is equal to the height h of the emittercylinder T, there are produced in the layers harmful interferences whichare propagated along the paths, such as 3 and 3. These interferences areaccentuated at the reception since the receiver R receives the entirebeam of acoustic rays returned at an angle r by the entire band of thewall with a width h equal to the height of the receiver cylinder R. Therays 4 and 5, 4 and 5' are the extreme rays of this beam in the plane ofFIG. 1. These defects become aggravated under the conditions which havealready been outlined above.

More particularly, even ifthe well is a cylinder of revolution, as inthe example shown in FIG. 1 wherein the axis of the sounding or probingtool or instrument does not coincide exactly with that of the well, theinterferences on the emitter-receiver paths are different for twodistinct paths as a result of the difference in length of the total pathbetween emitter and receiver; for example, for paths such as 2 3 5 and 13 4' (FIG. 1). The composition of the different signals received by thereceiver R will in actual practice render impossible the accurateinterpretation of the resulting signal.

In contrast thereto, in a device such as proposed by the presentinvention, as shown in FIGS. 2 and 2A, the emitter T and the receiver Rare disposed in a manner such that the mean cross-sectional plane M ofthe cylindrical emitting surface 7 and the mean crosssectional plane Mof the cylindrical receiving surface 8 are essentially coincident. FIG.2 represents an example in which the centers of the emitter and of thereceiver are not situated on the same vertical level, but thisdisposition or arrangement is by no means limiting.

Under these conditions, emitter T emits signals essentially followingthe radial directions of the cylindrical surface 7, and receiver Rreceives principally the signals directed along paths radial to thecylindrical surface 8. The receiver R receives but a single acoustic rayfor each acoustic impulse being emitted, and the signal which isreceived is, as a consequence thereof, much purer than that furnished bya sounding or prob ing tool as shown in FIG. 1.

In the case of FIG. 2 in which the emitter T and the receiver R aredisposed in proximity to a generatrix of the wall of the well, it willbe preferable, in order to receive only the rays issued from the wallclosest to the sounding instrument and for the purpose of avoidingthereby any risk of interference, to enclose the active cylinders of theemitter T and receiver R by means of a screen or cover C made from amaterial which absorbs the acoustic waves, without contact with theseactive cylinders, while providing in each screen or cover an openinglarge enough for the passage of the useful acoustic rays, such as theopenings 9 and 10, respectively, taking into account any variations ofthe angles i and r according to the geological formations. The materialused for making the screen or cover, which must also be able towithstand the hydrostatic pressure in the well, may, for example, byaraldite in which small lead balls are provided.

The studies of the propagation of acoustic waves in the terraintraversed by a drilling well between an emitter and a receiver locatedone above the other indicate the ability to distinguish in connectionwith each emitted wave one component corresponding to a vibration of therocks parallel to the axis of the well (longitudinal or compressionwave) and one component corresponding to a vibration of the rocksperpendicular to the axis of the well (transverse or shear wave).Between the wall of the well and the transducer (emitter T or receiverR), each of these components causes the mud or sludge filling the holeto vibrate according to a longitudinal mode of vibration.

FIGS. 3 and 3A show the application of the present invention to thedetermination of the dip angle of layers of terrain. The soundinginstrument illustrated therein comprises a sounding instrument body 11,shown in FIG. 3A, emitter transducers T T T and receiver transducers R RR The electronic emission and reception circuits have not been showntherein.

The body of the sounding instrument (FIG. 3A) comprises a centralcylindrical mandrel 12 made from a rigid material so that the speed ofthe acoustic waves in this material is lower than the speed of thesewaves in the drilling mud. Disposed on the aforementioned mandrel arefour centering blocks or supports, such as the support 13, above thesounding instrument, and four identical blocks or supports, such as thesupport 14, therebelow (FIG. 3A). There is disposed at the respectivelevels of the acoustic emitter transducers T T T and the acousticreceiver transducers R R R5 annular shoulder portions 15 and 16.Arranged in these shoulder portions are receptacles 17, 18, and 19 (FIG.3) for the emitter and receiver transducers. This portion of thesounding instrument body must be machined to have the followingcharacteristics: (1) the speed of the acoustic waves should be near thespeed of these waves in the liquid bathing the transducer cylinder and(2) an elevated acoustic damping coefficient should be obtained. Thesetwo requirements have the purpose of avoiding the parasitic return ofacoustic waves which would be reflected against the receptacle.

FIG. 3 represents a cross-sectional view through the body of thesounding instrument at the height or level of the shoulder portion 15which is disposed for the mounting of the emitters T T and T Thereceptacles l7, l8, and 19 may be closed by any suitable means whichwill not absorb the acoustic wave so that the drilling mud will notpenetrate thereinto. in this case, the transducers can be surrounded byan acoustic absorbing material which constitutes a cover for thisportion of the sounding instrument, except in a vertical sector where anacoustic window is provided which may either remain open or be closed bya material that will not essentially absorb the acoustic waves.

According to one embodiment of the present invention, the receptacle ofthe transducer is filled with oil. The material which constitutes theacoustic window must then be flexible enough to allow for theequalization of the pressures of the mud and of the oil bathing thetransducer. It must have a portion which is conductive at a referencepotential so as to assure the electrical insulation of the transducer.it must finally withstand corrosion, and could be, for example, anelastomer foil internally equipped with a conducting metallic lattice.

Another possible embodiment will be described hereinbelow with referenceto FIGS. 7 ane 7A, namely, an embodiment in which the receptacles of thetransducers are not closed, the mud thus being permitted to bathe thetransducers.

The active elements of the transducers are either solid cylinders (FlG.4) or hollow cylinders (FIGS. 5 and 6) disposed in such a manner thatthe axes thereof are horizontal; in other words, so that these axes arepositioned in a plane perpendicular to the drilling axis which isassumed to be vertical. The central vertical planes the three emitters TT and T coincide on the axis of the sounding instrument and will form atwo-bytwo dihedra enclosing angles of 120.

The annular shoulder portion 16 provided for the receivers has the sameshape or configuration as that enclosing or surrounding the emitters.The receivers are moreover mounted in the same manner as the emitters.Furthermore, the mean cross-sectional planes of the active cylinders ofemitter T and receiver R are coincident, like those of T and R T and RAs shown in H6. 3, the receptacles l7, l8, and 19 arranged in eachshoulder portion 115 and 16 have on both sides of each transducer arounded-off ridge, such as the ridges 20 and El projecting from thetransducer. These ridges have the function of protecting the transducers in case the sounding instrument should come to touch the wall ofthe well.

At each level of the well, the acoustic characteristics of the rocks arethus measured in three directions. The emission from emitters T T and Tis started simultaneously, and the corresponding signals being collectedat receivers R R and R respectively are recorded. The correlation of thediagraphs obtained, respectively, for the three pairs (T R (T R and (T Rmakes it possible to detect an inclination of the layers of terrain andto measure the amplitude thereof according to a technique which is wellknown in this field.

Whether they be emitters or receivers, the transducers may consist ofeither electrostrictive or magnetostrictive type devices (FIGS. 4' and5; and FIG. 6, respectively). The base material of the electrostrictivetransducers is a ceramic material available on the market, namely, amixture of zirconate and lead titanate with the addition of rare earths.The Curie point of such ceramic materials is higher than 300 C. Thisproperty assures a good behavior of the electrostrictive characteristicsto at least 200 C. and is thus suitable for the use of the transducersin petroleum wells where the temperature may reach this value at greatdepths.

The three emitters may have either identical or different emissionfrequencies. These frequencies may be comprised between 10 and 40 kHz.

When the active element of the emitter transducer is a solidelectrostrictive cylinder 24 (FIG. 4i), the electrical connections 29and 30 thereof are assured by the fastening of input and output wires onthe metallized plane faces 22 and 23 thereof.

Except in the sector where an acoustic window is disposed, thevibrations of the transducer are transmitted to its receptacle by thefluid in which this transducer bathes. These vibrations are absorbed bythe receptacle by virtue of the material which is chosen to make up orconstitute the receptacle. Particularly, the vibrations in a directionparallel to the axis of the cylinder are absorbed. The vibration emittedby the cylindrical surface 24 in a radial direction is therefore solelytransmitted to the exterior medium, this vibration reaching only theacoustic window. Likewise, only those vibrations will reach the receiverwhich traverse the acoustic receiv ing window. Thus, only thecylindrical surface 24 of each transducer will actually behave as anemitter or receiver, respectively.

When the transducer is a hollow electrostrictive cylinder 25 (FIG. 5),the input and the output wires 31 and/or 32 are connected to theinterior and to the exterior metallized faces 26 and, respectively, 27.When the transducer is of the magnetostrictive type (HO. 6), it consistsof a core 28, for example, made from iron cobalt or from iron silicon,on to which an electric wire 33 is wound. The ring is provided in a bandor strip'like winding of several hundredths millimeter thickness.

When this transducer configuration (hollow cylinder), and irrespectiveof whether this transducer is electrostrictive or magnetostrictive, itis significant tha the vibrations emitted by the interior cylindricalface will be damped acoustically. For this purpose, an acousticalabsorbing material must be placed at the inside of the transducer.

The embodiment of the present invention with the transducers beingbathed by the mud may be used in practice with transducers having theform of a hollow cylinder. When the transducer is of the piezoelectrictype shown in FIG. 5, the mounting or assembly thereof may beadvantageously made as shown in H68. 7 and 7A.

In this type of assembly, the ring 25 is secured between two supports orplates 34 and 35. The fluidtightness between the ring 25 and] eachsupport is assured by suitable annular seals 36 and 37. in this manner,the mud or sludge will not penetrate into the interior of the envelopeconstituted by the ring and the supports. The inner face of the supports34 and 35 is covered with a layer (38, 39) of acoustical absorbingmaterial. The securing or tightening of the supports may be assured bymeans of a screw 40. The interior of the envelope may contain either airor oil. In the latter case an equalization of the pressures prevailinginside and also on the outside may be obtained by using balancingmembranes 41 to 44. The exterior face of the supports 34 and 35 (faces45 and 46) is conductive. It may also be treated in the conventionalmanner so as to assure a protection against corrosion. The electricalconnections 31 and 32 may be made as shown herein. The high tension wire31 extends through one of the supports by means of a fluid-tight outputorifice 47. The high tension wire 31 and the wire to ground 32 areconnected to the electronic emission control system in the case of theemitters, and to the bottom or ground preamplifiers or input amplifiersin the case of the receivers, and specifically by means of fluid-tightterminals.

In the particular embodiment in which the active cylinder of thetransducers is of the magnetostrictive type (FIG. 6), the cylinder 28enclosed by the wire 33 can be clamped or clipped by winding of aconductive metallic band covered with an anti-corrosion material oragent, the unit then being placed directly in the drilling mud; themetallic band and the output wire of the coil is connected at one pointto the reference potential. The inputs and outputs of the wire 33 areobtained by using fluid-tight terminals.

The electronic circuits of the sounding instrument may be disposedeither within the body of the sounding instrument, or within fluid-tightcartridges adapted to the body of the sounding instrument. In the caseof a sounding instrument having the type shown in FIGS. 3

and 3A, the electronic emission circuits will be conceived in a mannersuch that the three emitters T T T are started successively and at arate which is variable by at least one impulse every second, which meansone impulse every 3 seconds for each emitter. There will be threeconductors present for putting together the signals, one for eachreceiver.

What is claimed is:

1. An acoustic sounding instrument for determining acousticalcharacteristics of the earth surrounding a borehole, comprising:

at least one acoustic emitter and at least one acoustic reciever spacedwith respect to each other,

said emitter including an active transducer element emitting acousticsignals principally radially from a curved, essentially cylindricalsurface of revolution, and said receiver including an actice transducerelement receiving the acoustic signals principally radially at a curved,essentially cylindrical surface of revolution,

said emitting cylindrical surface and said receiving cylindrical surfacebeing positioned with the mean cross-sectional planes thereof disposedessentially in the same longitudinal plane of said sounding instrument,wherein said emitter and said receiver each are enclosed in a cover madefrom a material capable of substantially absorbing the acoustic waves,said cover having at least one opening for the passage of the usefulacoustic rays to said active transducer elements.

2. An acoustic sounding instrument for determining acousticalcharacteristics of the earth surrounding a borehole comprising:

at least one acoustic emitter and at least one acoustic receiver spacedwith respect to each other,

said emitter including an active transducer element emitting acousticsignals principally radially from a curved, essentially cylindricalsurface of revolution, and said receiver including an active transducerelement receiving the acoustic signals principally radially at a curved,essentially cylindrical surface of revolution,

said emitting cylindrical surface and said receiving cylindrical surfacebeing positioned with the mean cross-sectional planes thereof disposedessentially in the same longitudinal plane of the sounding in strument,wherein said acoustic emitter comprises three emitters at a first levelof said sounding instrument, and said acoustic receiver comprises threereceivers at a second level of said sounding instrument, the meancross-sectional pleanse of respective emitters and receivers beingdisposed essentially in three planes forming therebetween angles of3..An acoustic sounding instrument for determining acousticalcharacteristics of the earth surrounding a borehole comprising:

at least one acoustic emitter and at least one acoustic receiver spacedwith respect to each other,

said emitter including an active transducer element emitting acousticsignals principally radially from a curved, essentially cylindricalsurface of revolution, and said receiver including an active transducerelement receiving the acoustic signals principally radially at a curved,essentially cylindrical surface of revolution,

said emitting cylindrical surface and said receiving cylindrical surfacebeing positioned with the mean cross-sectional planes thereof disposedessentially in the same longtiudinal plane of the sounding instrument,wherein said emitter and said receiver each are enclosed in a cover madefrom a material capable of substantially absorbing the acoustic waves,each cover having at least one opening for the passage of usefulacoustic rays to said active transducer elements, and wherein the activeelement of said emitter and that of said receiver are constituted of acylindrical ring secured axially between two circular plate meanscapable of substantially absorbing the acoustic waves and a pair offluid-tight annular seals disposed respectively between the cylindricalring and each of said plate means.

4. An acoustic sounding instrument according to claim 3, wherein theinner space delimited by said ring and by said plate means is filledwith an electrical insulating liquid, and in that at least oneequalizing membrane disposed in at least one of said plates assuresequality of hydrostatic pressure in the borehole and the pressure-ofsaid insulating liquid inside said transducer.

1. An acoustic sounding instrument for determining acousticalcharacteristics of the earth surrounding a borehole, comprising: atleast one acoustic emitter and at least one acoustic reciever spacedwith respect to each other, said emitter including an active transducerelement emitting acoustic signals principally radially from a curved,essentially cylindrical surface of revolution, and said receiverincluding an actice transducer element receiving the acoustic signalsprincipally radially at a curved, essentially cylindrical surface ofrevolution, said emitting cylindrical surface and said receivingcylindrical surface being positioned with the mean cross-sectionalplanes thereof disposed essentially in the same longitudinal plane ofsaid sounding instrument, wherein said emitter and said receiver eachare enclosed in a cover made from a material capable of substantiallyabsorbing the acoustic waves, said cover having at least one opening forthe passage of the useful acoustic rays to said active transducerelements.
 2. An acoustic sounding instrument for determining acousticalcharacteristics of the earth surrounding a borehole comprising: at leastone acoustic emitter and at least one acoustic receiver spaced withrespect to each other, said emitter including an active transducerelement emitting acoustic signals principally radially from a curved,essentially cylindrical surface of revolution, and said receiverincluding an active transducer element receiving the acoustic signalsprincipally radially at a curved, essentially cylindrical surface ofrevolution, said emitting cylindrical surface and said receivingcylindrical surface being positioned with the mean cross-sectionalplanes thereof disposed essentially in the same longitudinal plane ofthe sounding instrument, wherein said acoustic emitter comprises threeemitters at a first level of said sounding instrument, and said acousticreceiver comprises three receivers at a second level of said soundinginstrument, the mean cross-sectional pleanse of respective emitters andreceivers being disposed essentially in three planes formingtherebetween angles of 120*.
 3. An acoustic sounding instrument fordetermining acoustical characteristics of the earth surrounding aborehole comprising: at least one acoustic emitter and at least oneacoustic receiver spaced with respect to each other, said emitterincluding an active transducer element emitting acoustic signalsprincipally radially from a curved, essentially cylindrical surface ofrevolution, and said receiver including an active transducer elementreceiving the acoustic signals principally radially at a curved,essentially cylindrical surface of revolution, said emitting cylindricalsurface and said receiving cylindrical surface being positioned with themean cross-sectional planes thereof disposed essentially in the samelongtiudinal plane of the sounding instrument, wherein said emitter andsaid receiver each are enclosed in a cover made from a material capableof substantially absorbing the acoustic waves, each cover having atleast one opening for the passage of useful acoustic rays to said activetransducer elements, and wherein the active element of said emitter andthat of said receiver are constituted of a cylindrical ring securedaxially between two circular plate means capable of substantiallyabsorbing the acoustic waves and a pair of fluid-tight annular sealsdisposed respectively between the cylindrical ring and each of saidplate means.
 4. An acoustic sounding instrument according to claim 3,wherein the inner space delimited by said ring and by said plate meansis filled with an electrical insulating liquid, and in that at least oneequalizing membrane disposed in at least one of said plates assuresequality of hydrostatic pressure in the borehole and the pRessure ofsaid insulating liquid inside said transducer.